Chlor-Alkali Industry

Overview of Chlor-Alkali Industry Chlorine and co-product, caustic soda, fundamental building-block chemicals - Used in over 50% of all industrial chemical processes - Basic manufacturing chemical so products utilized in numerous industries and applications Essential to numerous products and services - Critical role in water disinfection and water delivery - 85% of all pharmaceuticals - 95% of محصول crop protection chemicals

سلعة Commodity Chemicals 1. Sulfuric Acid 2. Nitrogen 3. Oxygen 4. Ethylene 5. Lime (CaO) 6. Ammonia 7. Propylene 8. Phosphoric Acid 9. Caustic Soda 10. Chlorine 11. Sodium Carbonate 12. Ethylene Dichloride 13. Vinyl Chloride

Industry Description & Practices There are three basic processes for the manufacture of chlorine and caustic soda from محلول ملحي brine: the mercury cell, the diaphragm cell, and the membrane cell.

The membrane cell is the most modern and has economic and environmental advantages. The two other processes generate hazardous wastes (containing mercury or asbestos).

Chlor-Alkali Production by Process Type

In the membrane process, the chlorine (at the anode) and the hydrogen (at the cathode) are kept apart by a selective polymer membrane that allows the sodium ions to pass into the cathodic compartment and react with the hydroxyl ions to form caustic soda.

2NaCl + H2O → Cl2 + 2NaOH + H2 salt chlorine caustic soda hydrogen

The membrane cell process is the preferred process for new plants. Diaphragm processes may be acceptable, in some circumstances if non-asbestos diaphragms are used.

The energy consumption in a membrane cell process is of the order of 2,200-2,500 kiloWatt tons per metric ton compared to 2,400-2,700 kiloWatt tons per metric ton (kWh/t) of chlorine for a diaphragm cell process.

Mercury • Mercury cell process potential source of mercury emissions – Production processes vary by country/region – Continue to see transition from mercury cell process as plants reach the end of their economic life – Not a simple technology change, requires comprehensive conversion of entire plant – extremely capital intensive • Energy efficiency of mercury cell process – There is a difference in energy consumption between different processes, but partially compensated by increased energy to concentrate caustic soda to “commercial standard” – Potential energy savings depends on local electricity prices –Potential greenhouse gas reductions depends on local energy source

Electricity Consumption by Production Process

Waste Characteristics The major waste stream from the process is the “brine muds,” which are the sludges from the brine purification step and which may contain magnesium, calcium, iron, and other metal hydroxides, depending on the source and purity of the brines.

The muds are normally filtered or settled, the طافsupernatant is then recycled and the mud is dried and landfilled.

Chlorine is a highly toxic gas, and strict precautions are necessary to minimize risk to workers and possible releases during its handling.

Major sources of الهاربfugitive air emissions of chlorine and hydrogen are vents, seals, and transfer operations.

Pollution Prevention & Control The following pollution prevention measures should be considered: • Use metal rather than graphite anodes to reduce lead and chlorinated organics. • Resaturate brine in closed vessels to reduce the generation of salt sprays. • Use noncontact condensers to reduce process wastewater quantities.

• فرك Scrub chlorine tail gases to reduce chlorine discharges and to produce hypochlorite. • Recycle condensates and waste process water to the brine system, if possible. • Recycle brine wastes, if possible.

For the chlor-alkali industry, an Emergency Preparedness and Response Plan is required for potential uncontrolled chlorine and other releases.

Carbon tetrachloride is sometimes used to scrub nitrogen trichloride (formed in the process) and maintain its levels below 4 percent to avoid explosion.

However, substitutes for carbon tetrachloride may have to be used as the use of carbon tetrachloride may be banned globally.

Treatment Technologies Caustic scrubber systems should be installed to control chlorine emissions from condensers and at storage and transfer points for liquid chlorine.

Sulfuric acid used for drying chlorine should be neutralized before discharge.

Brine muds should be discharged to lined settling ponds (or equivalent) to prevent contamination of soil and groundwater. Effluents should be controlled for pH by neutralization.

Settling and filtration are performed to control total suspended solids. Dechlorination of wastewaters is performed using sulfur dioxide or bisulfite.

Emission Guidelines Air Emissions Chlorine concentration should be less than 3 milligrams per normal cubic meter (mg/Nm3) for process areas including chlorine liquefaction.

Liquid Effluents For membrane cell effluents, pH levels should be in the range 6-9. For non-asbestos diaphragm plants, the following effluents levels should be achieved; In some cases, bioassay testing of effluents may be desirable to ensure effluent toxicity is at acceptable levels, say toxicity to fish at a dilution factor of two.

Ambient Noise Noise خفض abatement measures should achieve either the following levels or a maximum increase in background levels of 3 dB(A). Measurements are to be taken at noise receptors located outside the project property boundary.

Monitoring & Reporting Frequent sampling may be required during start-up and upset conditions. Daily monitoring for parameters other than pH (for effluents from diaphragm process) is recommended. The pH in the liquid effluent should be monitored continuously. Chlorine monitors should be strategically located within the plant to detect chlorine releases or leaks on a continuous basis.

Monitoring data should be analyzed and reviewed at regular intervals and compared with the operating standards so that any necessary corrective actions can be taken.

Key Issues The following box summarizes the key production and control practices that will lead to compliance with emissions guidelines: